A. Field of Invention
The present invention relates generally to a plasma torch and, more particularly, to a connection between a plasma torch electrode, nozzle, and a retaining member.
B. Description of the Related Art
Commonly used for working of metals, plasma arc torches are used for cutting, welding, surface treatment, melting, and annealing. These torches include an electrode that supports an arc that extends from the electrode to the workpiece in the transferred arc mode of operation. It is also conventional to surround the arc with a swirling vortex of gas, and in some torch designs, it is conventional to envelope the gas and arc with a swirling jet of water.
It is known in the art of metal inert gas (mig) welding, that the welding gun contact tip must satisfactorily perform two important functions. First, it must accurately guide a weld wire to the point of contact with the work piece. Second, it must conduct adequate current, which can be as high as 400 amps or more, to the weld wire. To achieve those results, it is known to provide a relatively long contact tip made of copper and having an axial hole therethrough. The hole is sized slightly larger than the diameter of the weld wire. The contact tip extends from a gas diffuser through which an inert gas is directed around the welding arc. Current to the contact tip is supplied to it through the gas diffuser.
Due to the severe operating conditions, contact tips have relatively short service lives. Consequently, they are treated as consumable items in the industry. Typically, contact tips are made as separate pieces that are threaded into a gas diffuser. The thread is a single thread and, therefore, each contact tip has only one seating location with respect to the gas diffuser. When a contact tip has worn, it is threaded out of the gas diffuser, is discarded, and a new one is screwed into its seating location.
As noted above, in addition to heating caused by the welding arc, another potential source of heat to the contact tip is the threads between it and the gas diffuser. That is, the threads can cause resistance heating. To minimize electrical resistance, the contact tip and gas diffuser threads have a relatively long engagement length and a fine pitch. However, the long engagement, fine pitch, thread presents a problem. Removing a worn contact tip from the gas diffuser requires several turns of the contact tip. If the weld wire has burned back and fused to the contact tip, the weld wire must twist through the same number of turns as the contact tip. The potential springback of the fully removed contact tip due to the twisted weld wire is annoying if not potentially dangerous. Moreover, the time required to disassemble the multiple-turn contact tip also contributes to downtime of the welding production area.
Although the prior art has individually addressed the problems of short service life of a contact tip, resistance heating, and quick replacement of a contact tip, there has not been one convenient configuration that solves all of these problems.
U.S. Pat. No. 5,440,100 to Stuart et al. and U.S. Pat. No. 3,469,070 to Bernard et al. each discloses a contact tip that can be seated in two different positions within a gas diffuser. Stuart and Bernard use a projection on the gas diffuser, wherein the projection fits into a groove on the contact tip. But such a connection is weak both mechanically, and electrically. Mechanically, there is only a point contact between the gas diffuser and contact tip that holds them together. In Stuart, a dimple or protrusion extends from the gas diffuser; however, the dimple is subject to wear each time a contact tip is inserted within the gas diffuser thereby making the mechanical connection loose over time. In Bernard, a set screw in the gas diffuser extends into a bore that receives the contact tip. The tip of the set screw fits into recessed guideways on the contact tip. But the set screw may loosen due to vibration during use of the welding gun and, therefore, weaken the mechanical connection between the contact tip and gas diffuser. Further, the point contact tends to loosen easily thereby allowing the contact tip to rattle within, and quite possibly come out of, the gas diffuser, which would be a big problem during welding operations. Electrically, each Stuart and Bernard suffers the drawback of resistance heating near the limited point contact between the projection and the groove that hold the parts together.
U.S. Pat. No. 5,760,373 to Colling addresses the problem of resistance heating by providing a welding device having a tapered connection between a contact tip and a gas diffuser. However, Colling still suffers the drawback of requiring a full turn of the contact tip to remove or insert it within the gas diffuser. Moreover, Colling's contact tip seats in only one position relative to the gas diffuser and, therefore, suffers the additional drawback of a short service life.
Similarly to Colling, U.S. Pat. No. 5,726,420 to Lajoie provides a welding device having a tapered connection between a contact tip and a gas diffuser. Lajoie claims to enable quick replacement of the contact tip, however, Lajoie's device requires 2½ turns of the contact tip relative to the gas diffuser in order to remove or insert it. Lajoie's required 2½ turns is still quite time consuming and, therefore, contributes to a long downtime during replacement of the contact tip. Further, Lajoie's electrode seats in only one position relative to the gas diffuser and, therefore, suffers the additional drawback of a short service life.
U.S. Pat. No. 5,023,425 (Severance, Jr.) which issued on Jun. 11, 1991, and which is incorporated herein by reference, discloses an electrode for a plasma arc torch wherein the electrode includes a copper holder having a lower end that mounts an emissive insert that acts as the cathode terminal for the arc during operation.